The present invention relates to die casting machines and particularly to a system for adjusting the tie bar tension.
In die casting machines which frequently operate on an almost continuous basis during operation, it is important that the tie bar tension remain within preselected limited to maintain quality castings and to prevent excessive wear on dies and the machine itself. During operation of die casting machines, even if the initial preset tie bar tension is accurately provided for the desired operation of the machine, the tie bar tension varies due largely to the heating effect of molten shot on the dies. Thus, for example, a cold machine is set up for a lockup force depending upon the size of the machine and the material being cast but which, for example, might be in the order of 2000 tons total with 500 tons being shared for each of the four tie bars typically employed in a die casting machine. During use of the machine where molten metal is injected into dies, the machine gradually heats. It has been found that for a typical 72 inch die, for example, a temperature rise of 100.degree. F. can increase the die thickness by as much as 0.045 inch. This can cause the preselected tensil force on the tie bars to change dramatically resulting in uneven closing of the die in the event the tension shared by the tie bars becomes significantly uneven or undesired lockup forces result, causing bad castings. The casting metal then can flash from the mold and coin into the die face which also shortens the die life.
With conventional die casting machines, although monitoring systems to determine tie bar tension have been used, it is necessary to manually and individually adjust each of the tie bars during the course of a day's operation to assure casting quality remains relatively constant with changing temperatures and, thus, tension on the tie bars of the machine. This, naturally, shuts down operation of the machinery and is costly in terms of lost production and use of manpower.